1. Field of the Invention
This invention relates in general to the field of spectral shift, pressurized light water nuclear reactors and in particular to a reactor internals system for distributing the flow of a low neutron fluid moderator to the core of the reactor to achieve the spectral shift.
2. Description of the Prior Art
In conventional, state of the art, pressurized light water nuclear reactors, the reactor core is designed to contain excess reactivity. As the reactor operates, the excess reactivity is very gradually consumed until such point as the reactor core will no longer sustain the nuclear reaction and then the reactor must be refueled. Usually this occurs over a period of years. It is very advantageous to maximize the time between reactor refuelings (extend the life of the core) since refueling requires complete shutdown of the reactor and is quite time consuming. Extending the life of the core is usually accomplished by providing the core with a significant amount of excess reactivity.
Typically, control over the fission process, or reactivity control, including control necessitated by the excess reactivity is accomplished by varying the amount of neutron-absorbing materials within the core of the reactor. Control rods which contain neutron-absorbing materials and are movable into and out of the core provide one method of controlling the reactivity. Burnable and nonburnable poisons dissolved in the reactor coolant provide another method of reactivity control. As the reactivity decreases, due to reactor operation, the poisons are gardually removed by being burned by reactor operation or are physically removed by a separate system designed for such purpose. Most often, a combination of dissolved poisons and control rods are used to control the reactor and the excess reactivity.
Unfortunately, control with control rods and poisons, absorb neutrons which could otherwise be used in a productive manner. For example, the neutrons produced by the excess reactivity could be used to convert fertile materials within the fuel assemblies to plutonium or fissile uranium which can then be fissioned and contribute to an even further extension of core life. Thus, while the use of control rods and dissolved poisons provide very effective reactor control, their use comprises a relatively inefficient depletion of high cost uranium. It would be, therefore, advantageous to control the excess reactivity, but not suppress the neutrons associated with the excess reactivity, in order to further extend core life or time between refuelings, and to lower fuel costs.
It is known that fuel element enrichment can be reduced and the conversion ratio of producing fissile materials can be increased by employing a "hardened" (nuclear energy) spectrum during the first part of the fuel cycle to reduce excessive reactivity and to increase the conversion of fertile material to fissile material; then employing a "softer" (lower energy) neutron spectrum during the latter part of the fuel cycle to increase reactivity and extend the core lift by fissioning the previously generated fissile material. One such method utilizing the above is known as spectral shift control which provides a reactor with an extended core life while reducing the amount of neutron-absorbing material in the reactor core. One example of such method of control comprises a mechanical spectral shift reactor whereby hollow displacer rods are provided within fuel assemblies within the core (which, of course, displace an equal volume of water within the fuel assemblies) and which are mechanically withdrawn or punctured to accomplish water flooding of the available volume. In the early stages of core life, the neutron spectrum is hardened by the displacement of a portion of the water within the core by the displacer rods. The spectrum is later softened by the addition of water within the core by the aforesaid rod withdrawal or puncturing. Patent application Ser. No. 217,054 now U.S. Pat. No. 4,432,930 entitled " Spectral Shift Reactor Control Method" by A. J. Impink, Jr., et al., filed on Dec. 16, 1980, assigned to Westinghouse Electric Corporation, discloses one such mechanical spectral shift reactor.
Another method of achieving a spectral shift is to utilize heavy water or deuterium oxide to replace an equivalent volume of core water during the early stages of core life then to gradually reduce the volume of heavy water and replace it with regular reactor coolant (light water) during the later stages of core life. The less effective moderator, heavy water, allows for less fuel enrichment and a higher ratio of converting fertile material to fissile material which in combination provides for a reduction of fuel costs and an extension of core life. An example of this art is found in patent application Ser. No. 626,847 entitled "Fuel Assembly" by R. K. Gjertsen, et al., filed on July 2, 1984 (W.E. No. 49,102) and assigned to Westinghouse Electric Corporation.
In the "Fuel Assembly" patent application, there is explained the need to introduce heavy water into the core support plate for distribution to the fuel assemblies and for eventually exiting the heavy water from the fuel assemblies and through and out of the core support plate.
However, while such requirements of a spectral shift nuclear reactor are well known, to date no apparatus exists which effectively and practically accomplishes such requirements. Also, there is a corresponding need to introduce and exit the deuterium oxide into and out of the pressure vessel. Again, no apparatus to accomplish the same is presently known to exist.
It is, therefore, a primary object of the present invention to provide a distribution system for introducing into the reactor pressure vessel and into the core a moderator which is a less effective moderator than the normal reactor coolant in the early stages of core life and which permits the gradual replacement of the low neutron moderating fluid with the normal reactor coolant during the later stages of core life.